Code review comment for lp:~vanvugt/mir/fix-1408906

pre-existing, but for some android driver versions we can't seperate flip() from the gl rendering (and won't really have an accurate time)

okay by me, pre-existing nit:
39 + virtual void rendered_frame(SubCompositorId id = 0) = 0;
40 + virtual void finished_frame(SubCompositorId id = 0) = 0;
default parameters
http://unity.ubuntu.com/mir/cppguide/index.html?showone=Default_Arguments#Default_Arguments